A Volcano is a gap in the earth where molten rock and other materials come to the earth's surface. Some volcanoes are just cracks in the earth's crusts. Others are weak places in the
earth's crust, which occur on places where magma bubbles up through the crust and comes to the earth's surface. Magma is molten rock that occurs by partial melting of the crust and the mantle by
high temperatures deep down in the ground. Once magma comes to the earth's surface it is called lava.

Active and non-active volcanoes

There are volcanoes in different phases of activity:Active volcanoes, which are likely to erupt at any moment, dormant volcanoes, which lie dormant for centuries, but then erupt
suddenly and violently, and extinct volcanoes - ones no longer likely to erupt.

Types of volcanoes

In the surroundings of boundaries of tectonic plates the following types of volcanoes occur:

The fissure volcano

Is a long crack in the earth's surface through which magma erupts. These cracks may form as two tectonic plates pull apart.You'll find them mainly near mid-ocean ridges

The shield volcano

This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly.

The dome volcano

This one has a steep, convex slope from thick, fast-cooling lava.

The ash-cinder volcano

Throws out - besides lava - much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder.

The composite volcano

These are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope.

The caldera volcano

An older volcano with a large crater which can be 62 miles(100km) wide. In this crater many little new craters are formed.